The present invention relates to a method for generating video holograms for a holographic display device with random addressing.
The means for implementing the method will be referred to as content generation means. According to the method, the content generation means generate the hologram values which are required for the representation in the holographic display device, and can thus be interpreted as data source. The generated hologram values can be transmitted directly to the holographic display device or stored in digital storage means.
A holographic display device with random addressing is interpreted as data receiver and provides the holographic representation of the hologram data which are generated according to the method or the content generation means.
Real-time reconstruction of holograms has proved its suitability in many important applications thanks to the progress made in hardware components and computation methods. A major challenge in digital holography is to cope with the amount of data per image, which is much larger than that of conventional video information. This large amount of data makes great demands on storage media and data transmission means such as network components and bus systems. Already the transmission and processing of conventional video data makes great demands on those resources.
A holographic display device is substantially based on the principle that a sub-hologram is defined together with each object point of the scene to be reconstructed and that the entire hologram is formed by a superposition of sub-holograms, with the help of at least one light modulator means on which a scene which is divided into object points is encoded as an entire hologram and where the scene can be seen as a reconstruction from a visibility region which lies within one periodicity interval of the reconstruction of the video hologram. In general, the principle is to reconstruct mainly that wave front that would be emitted by an object into one or multiple visibility regions.
In detail, such a device is based on the principle that the reconstruction of an individual object point only requires a sub-hologram as a subset of the entire hologram encoded on the light modulator means. The holographic display device comprises at least one screen means. The screen means is either the light modulator itself, where the hologram of a scene is encoded, or an optical element—such as a lens or a mirror—onto which a hologram or wave front of a scene encoded on the light modulator is projected.
In this document, the term ‘light modulator means’ or ‘SLM’ denotes a device for controlling intensity, colour and/or phase of light by way of switching, gating or modulating light beams emitted by one or multiple independent light sources. A holographic display device typically comprises a matrix of controllable pixels, which reconstruct object points by modifying the amplitude and/or phase of light which passes through the display panel. A light modulator means comprises such a matrix. The light modulator means may for example be an acousto-optic modulator AOM or a continuous-type modulator. One embodiment for the reconstruction of the holograms by way of amplitude modulation can take advantage of a liquid crystal display (LCD). The present invention also relates to further controllable devices which are used to modulate sufficiently coherent light into a light wave front or into a light wave contour.
The term ‘pixel’ denotes a controllable hologram pixel of the light modulator, it represents a discrete value of the hologram point and is addressed and controlled discretely. Each pixel represents a hologram point of the hologram. In the case of an LC display, a pixel is a discretely controllable display pixel. In the case of a DMD (Digital Micro-mirror Device), such as a DLP (Digital Light Processor), a pixel is a discretely controllable micro-mirror or small group of such mirrors. In the case of a continuous light modulator means, a pixel is an imaginary region which represents the hologram point. In the case of a colour representation, a pixel is typically sub-divided into multiple sub-pixels, which represent the primary colours.
The term ‘transformation’ shall be construed such to include any mathematical or computational technique which is identical to or which approximates a transformation. Transformations in a mathematical sense are merely approximations of physical processes, which are described more precisely by the Maxwellian wave equations. Transformations such as Fresnel transformations or the special group of transformations which are known as Fourier transformations, describe second-order approximations. Transformations are usually represented by algebraic and non-differential equations and can therefore be handled efficiently and at high performance using known computing means. Moreover, they can be modelled precisely using optical systems.
The transfer of image data from the content generation units, i.e. data source and visualisation module (e.g. an LCD or CRT monitor as data receiver) conventionally works such that the entire content of an image is output line by line from top to bottom, as with conventional tube monitors. This does not pose a problem for HDTV monitor resolutions, because the required amount of data can be transferred fast enough through standardised interfaces, for example Digital Visual Interface (DVI) or High Definition Multimedia Interface (HDMI).
The means for the content generation, i.e. the data source, are for example graphics cards or graphics sub-systems where the 3D rendering pipeline is implemented. A 3D rendering graphics pipeline describes the way from the vectorial, mathematical description of a three-dimensional scene to pixelated image data in a frame buffer in order to be displayed on a monitor screen. The three-dimensional image data comprise depth information and usually also additional information about material and surface properties. For example, the conversion of screen coordinates into device coordinates, texturing, clipping and anti-aliasing are performed in that pipeline. The pixelated image, which represents a two-dimensional projection of the three-dimensional scene, and which is stored in the frame buffer of a graphics adapter, comprises the pixel values for the controllable pixels of a monitor screen, for example an LC display. The holographic pipeline generates the complex hologram values for the representation on the holographic display device from the results of the 3D rendering graphics pipeline.
Holographic display devices require a much larger number of pixels than a conventional two-dimensional display, and thus a much larger amount of data for the hologram values. The object of the present invention is to provide a method which significantly reduces the generated amount of data for video holograms. The amount of data which is to be transferred between the data source, i.e. content generation means, and the data receiver, i.e. the holographic display device, shall thus be minimised. Consequently, the amount of data which is required to be stored for the video holograms in digital storage media shall also be reduced. Further, a holographic display device shall be provided which ensures that the sequence is represented without a loss of quality also with that reduced amount of data. The method shall take advantage of known means for data storage and transfer. A manageable amount of data shall contribute to improving acceptance and distribution of video holograms.
During the transfer as described in the prior art, the entire amount of data is transferred per image or video frame of a sequence from the content generation means, i.e. the data source, to a holographic displays device, i.e. the data receiver. This means that the entire hologram information is transferred, including those pieces of information which do not change from one image to the next one. Because a hologram reconstructs object points in a three-dimensional space, it is sufficient to know which object points have changed in a video frame compared to the previous video frame. The modification particularly relates to the position, but also to the colour and intensity.
The method for generating video holograms according to this invention is particularly suited for holographic display devices with at least one light modulator means on which a scene which is divided into object points is encoded as an entire hologram and where the scene can be seen as a reconstruction from a visibility region which lies within one periodicity interval of the reconstruction of the video hologram, where a sub-hologram is defined by the visibility region together with each object point of the scene to be reconstructed and where the entire hologram is formed by a superposition of sub-holograms. Such a holographic display device with light modulator means is based on the principle that the wave fronts which are modulated with the information of object points of a scene are superposed in at least one visibility region. The definition of a visibility region has already been given above.
Further, advantage is taken of the principle that the reconstruction of an individual object point of a scene only requires a sub-hologram as a subset of the entire hologram encoded on the light modulator means. Each single object point is created by one sub-hologram, whose position depends on the position of the object point, and whose size depends on the observer position. The region of the sub-hologram on the light modulator means will be referred to below as modulator region. The modulator region is that sub-region of the light modulator means which is required for reconstructing the object point. At the same time, the modulator region defines which pixels on the light modulator must be addressed in order to reconstruct that object point. The modulator region will remain in a fixed position if the object point is an object point which is fixed in space. This means that the object point to be reconstructed changes its position depending on the observer position. A change of the modulator region in dependence on the observer position allows the object point to be encoded at a fix position, i.e. its position in space does not change depending on the observer position. As far as the present invention is concerned, those principles can be applied analogously. According to a most simple embodiment, the centre of the modulator region lies on the straight line which runs through the object point to be reconstructed and the centre of the visibility region. In a most simple embodiment, the size of the modulator region is determined based on the theorem of intersecting lines, where the visibility region is traced back through the object point to be reconstructed to the light modulator means.
Also if sub-holograms are preferably used, a pixel, which represents the smallest controllable unit of the light modulator, does not only comprise the information of a single sub-hologram, but, as a result of the superpositions, the information of multiple sub-holograms.